TV modulators are somewhat similar to AM modulators, but the video signal is "single ended" instead of "dual ended", like audio. The video signal has to get modulated such that the TV channel carrier is at peak amplitude on sync tips, and around 5% carrier level on peak white. The input video should be "clamped", aka DC restored right at the modulator tube/transistor. You don't really have to do anything extra for color. As for the sound, run an FM modulator at 4.5MHz and add that to the video just before the TV modulator. The video signal may need a 4.5MHz notch filter to avoid video energy at 4.5Mhz from interfering with the sound carrier. Though FM can take a fair amount of interference and not be bothered.
If you have an old notch filter color TV, and a DVD player with an S video output jack, you can get better picture quality by passing the luma video thru a 3.58 band stop notch filter, and the chroma signal thru a 3.58 band pass filter, and then adding them together to make a compsite signal. Then feed that to a TV channel modulator.
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